How do I check if string::append() is going to fail

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C++

Windows

I am writing a code in which I need to create a std::string by appending data to it(here data means logs of an application). But string also has its limitation of size. I have written the code in try-catch block and when append() fails, it throws exception. I need to check before append() whether attaching new string will cause any exception, since once exception is thrown, it clears the data from string.
I want to check if attaching new string will cause any exception and if yes then write the data to a file and clear the string for next input.
As it is an application log, I do not know what the final size will be. So I am trying to make code less vulnerable to above explained problem.
I have checked one condition as follows:
If "current_string_size + to_be_attached_string_size exceeds string.max_size()" then write the code into file.
If "not" then append to_be_attached_string to current_string.
But it is failing somewhere.
What condition I need to check?